Durum wheat (Triticum turgidum L. var. durum) accumulates Cd from the soil depending on various factors. When grown in hydroponic solution containing Cd (20 microg l(-1)), roots had higher tissue Cd concentrations than shoots or heads. Kyle (the higher grain-Cd accumulating cultivar) had lower root-Cd, and greater shoot-Cd and head-Cd concentrations than Arcola (the lower grain-Cd accumulating cultivar). These cultivar differences were greater at flowering and ripening than at tillering. Much of the root-Cd was lost between the flowering and ripening stages of development. Distribution of (106)Cd among plant parts, after a single 24 h feeding, demonstrated that root-to-shoot transfer of Cd in Arcola was similar to that of Kyle at tillering, but it had ceased at flowering in Arcola but not Kyle. None of the Cd in wheat heads at ripening originated from (106)Cd exposure in the previous 24 h, suggesting that grain-Cd is a function of total shoot accumulation. Both cultivars demonstrated basipetal translocation of Cd; Arcola at tillering translocated more Cd from shoots to roots than Kyle. The proportion of Cd(2+)/Cd(total) in the nutrient solution decreased with time, suggesting that plant activity altered the solution chemistry. The alteration probably resulted from either preferential depletion of solution Cd(2+) and/or addition of root exudates. Lower grain-Cd accumulation in Arcola possibly resulted from a combination of reduced root-to-shoot transfer of Cd at flowering, as well as enhanced shoot-to-root retranslocation of Cd, at least in younger plants. Plant-mediated changes in solution-Cd speciation did not play a role.

The objective of the study was to compare in vivo estimates of Cd bioavailability in two diet materials (lettuce and durum wheat grain) with bioaccessibility estimates from three in vitro methods. For both dietary materials, the Cd was either incorporated during growth or applied topically as a soluble salt just prior to experimentation. Simulated gastric/intestinal digestion using a physiologically based extraction technique (PBET) solubilized less than 56% (lettuce) or 13% (grain) of the Cd that was either incorporated into the plant tissues during growth, or added to the plant tissues before experimentation, as Cd(NO3)2.H2O. Amended diets could not be distinguished from incorporated diets. More of the Cd solubilized from amended lettuce than from incorporated lettuce moved to the outside of MWCO 10 kD and 25 kD dialysis sacs; no difference between the amended and incorporated diets was observed for grain. The percentage of lettuce-Cd solubilized by the PBET and sorbed by Caco-2 cells was greater for incorporated than for amended lettuce; for Cd in grain, the reverse occurred. As expected, none of the in vitro estimates of bioaccessibility were the same percentage of Cd in the lettuce or grain as was measured as bioavailable in vivo. The in vitro assays all predicted that substantially less than 100% of the Cd in the foods would be bioavailable, as was identified in vivo, and simulating intestinal selectivity improved the comparison to in vivo. Some of the in vitro assays identified subtle differences between the diets (i.e., amended vs. incorporated) that were consistent with in vivo studies, and with speculated differences in Cd speciation; this suggests their potential usefulness for the study of modifiers to dietary Cd bioavailability.

Quantifying the transfer of Cd from foods to mammalian target organs is key to estimating the health risk from this exposure; however, the bioaccumulation of Cd from foods is modified by many dietary components. Studies of dietary Cd absorption would be simpler if it were known that Cd added to foods as a soluble salt was as bioavailable as Cd incorporated during growth of the food species. Rabbits were fed, for 16 d, fresh lettuce containing cadmium incorporated into the lettuce during growth or added to the lettuce before feeding, or lettuce with no Cd but soluble Cd administered to the animals by gavage. There was a marked positive relationship between increased Cd dose and its accumulation in kidney; the slopes for the gavage and added treatments were not clearly different from the incorporated treatment; liver data were highly variable. In a 10-wk study of Cd-incorporated and -amended lettuce diets, for the incorporated and control diets there was less Cd accumulation in the kidneys, but not liver, per unit cumulative dose, than for the amended diet. Cd accumulation in the small intestine and Cd concentration in feces, both per unit daily dose, were smaller for the incorporated than for the control and amended diets; Cd concentrations in bile, urine, and serum, per unit daily dose, were higher in the control diet than values in the amended diet, which were higher than the incorporated diet. These differences could not be accounted for by variation in Fe or Zn contents of the diets. Thus, data suggest that Cd-amended diets overestimate bioaccumulation in kidney, an important target organ, by up to one-third, and that studies of short duration are not adequate to evaluate Cd bioavailability from food.

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